Capacitive measurement and/or detection systems have a wide range of applications, and are among others widely used for the detection of the presence and/or the position of conductive body in the vicinity of an electrode of the system. A capacitive sensor, called by some electric field sensor or proximity sensor, designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person's body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which could comprise the one or more antenna electrodes themselves—at which the influence of an object or living being on the electric field is detected.
The technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith, published in Computer Graphics I/O Devices, Issue May/June 1998, pp 54-60 describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, which is alternatively referred to as “coupling mode”, an oscillating voltage signal is applied to the transmit electrode, building up an electric field to a receive electrode, and the displacement current induced at the receive electrode is measured, whereby the displacement current may be modified by the body being sensed. In the “transmit mode”, the transmit electrode is put in contact with the user's body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
The capacitive coupling is generally determined by applying an alternating voltage signal to a capacitive antenna electrode and by measuring the current flowing from said antenna electrode either towards ground (in the loading mode) or into the second electrode (receiving electrode) in case of the coupling mode. This current is usually measured by means of a transimpedance amplifier, which is connected to the sensing electrode and which converts a current flowing into said sensing electrode into a voltage, which is proportional to the current flowing into the electrode.
While the above measurement principle generally leads to very useful results, it is clear that problems may arise in the vicinity of grounded structures (e.g. seat heaters in a vehicle seat). A seat heater comprises a heating element, which is typically a low-resistance conductor (in the form of a wire, cable, conductive trace printed on an insulating substrate, or the like) for being arranged under the seat cover.
The base capacitance measured between the sensing electrode of a capacitive occupant detection system and the reference electrode (car body) increases in close vicinity of a seat heater structure especially for systems without guard/shield electrode. This is due to the additional capacitive coupling between the sensing electrode and the seat heater structure, which by itself has a low impedance connection to the reference electrode (car body ground). Accordingly it is normally not possible to clearly separate the capacitance influence of the seat heater structure from the influence of the remaining car body since for example the seat heater structure is usually a separate unit. As a further effect, this capacitance is subject to tolerances due to (assembly-related) variations in distance between sensing electrode and seat heater structure, due to ageing effects of the composite material between the sense electrode and the seat heater, as well as modifications of physical parameters of said composite material (e.g. permittivity). This increased capacitance and the related tolerances affect the useful dynamics of a connected measurement system and limit the accuracy of decisional thresholds and the robustness of these thresholds. Finally as there is also a capacitive coupling between the sensing electrode and the seat frame, said seat frame's grounding state needs to be also controlled to avoid variance in the capacitive load.
In the current state of the art, the seat heater influence and the seat frame influence might be reduced by the use of a guard/shield electrode between the sense electrode and the seat heater or by active functional coupling with a seat heater electronics module which is controlled by the capacitive measurement system. These current implementations increase system cost (additional large-area guard electrode) and/or complexity (interaction with seat heater module).